1. Field of the Invention
The present invention relates to a method for annealing semiconductors, in particular, silicon (Si), germanium (Ge), gallium arsenide (GaAs) and so forth.
2. Description of the Prior Art
At present, annealing is drawing the attention of the semiconductor industry in two aspects. One is for the restoration of semiconductor crystals from damages which are caused, for example, by ion implantation of boron and the other is the use of high energy into a silicon wafer with a view to crystallization of a semiconductor element and providing it with a novel function. The most common traditional way of annealing is what is called the electric furnace method, in which the wafer is heated in a furnace, for instance, at 1000.degree. C. for 30 minutes while supplying thereinto dry nitrogen. This method is simple, but it has such defects as follows:
(a) it causes warps in the wafer, thus affecting the productive yield in the ensuing processes;
(b) as heating requires a prolonged time, the interior of the wafer is subject to changes in the ion distribution;
(c) the surface of the wafer is apt to be contaminated; and
(d) prolonged time is needed for annealing. In view of these defects, alternatives therefor are now being sought, e.g., by the application of laser rays for brief exposure of the subject wafer thereto. Laser ray annealing, however, such as in the case of using a pulse-oscillation laser, also has the following defects:
(e) the distribution of the implanted ions is subject to substantial changes as their diffusion rate is extremely high in the liquid phase, though the crystal restoration is effected because of the fusion of the wafer surface and the achievement of its crystallization through the liquid epitaxial growth;
(f) because the radiated light is of a single wavelength, there occurs an interference pattern in the melting area, leading to the uneven irradiation on the wafer;
(g) in the case of emloying a continuous oscillation laser, it means the scanning of a small beam spot on the wafer, resulting in producing a portion wanting sufficient annealing in the linear boundaries between scanning lines, and if the space between the scanning lines is reduced, the scanning takes much time and often yields overheated portions, thus causing such a disadvantage as uneven irradiation; and
(h) because of the laser light being of a single wavelength, an interference pattern is developed on the wafer surface to cause uneven irradiation, and also as a common defect of the laser ray annealing methods, they call for large and precise equipments and further advanced techniques for operation.
The other annealing is for producing, for instance, a silicon wafer by the epitaxial growth of a silicon layer which is deposited on a suitable substrate by means of the ion evaporation technique. The annealing, in such a case, was hitherto performed in the same manner as above mentioned, that is, in an electric furnace, or by exposing the wafer to the laser ray, and in this case, there are the same defects as previously mentioned.